This invention concerns control of a continuous roll caster of a type commonly used for casting aluminum base alloys. One type of roll casting machine is described in U.S. Pat. No. 4,054,173 by Hickam, the subject matter of which is hereby incorporated by reference.
In such an apparatus a pair of water cooled parallel casting rolls are positioned one above the other. These rolls are spaced apart a distance corresponding to the thickness of a sheet being cast. A pouring tip fits snugly into the converging space between the casting rolls on the entrance side for introducing molten metal into the nip of the rolls. In an exemplary caster each of the rolls is about 1 meter in diameter and they have a length in the order of 1.5 to 1.8 meters.
Preferably the plane in which the rolls axes lie is not vertical, but instead is tilted backward by about 15 degrees. That is, the plane is tilted so that the upper roll is about 15 degrees nearer the entrance side than the lower roll. The metal thus tends to move somewhat upwardly into the nip of the rolls. This is referred to as a tilt caster. A so called horizontal caster has the rolls in a vertical plane with metal flowing horizontally into the nip of the rolls. Other casters for aluminum have the rolls in a horizontal plane with metal flowing vertically into the rolls.
The rolls are motor driven so that a cast sheet is extruded from the exit side of the casting machine. Typically, this sheet is conveyed to a coiler that forms a tight coil of sheet for transport to subsequent processing. The rolls are rotated slowly so that sheet is cast at a rate less than about two meters per minute.
Some roll casting machines are made with a single motor driving the two rolls in synchronism with each other. This requires that the rolls have carefully proportioned diameters to maintain the desired proportionality of surface speed of the two rolls. The two rolls must turn at almost the same speed to successfully cast flat sheet.
Some roll casting machines are made with separate motor drives for each of the two rolls. This permits independent speed control of the two rolls so that different roll diameters can be accommodated. This can be an appreciable economy in maintaining the rolls.
For example, it is commonly observed that the bottom roll in a caster has a greater amount of heat checking and other surface degradation than the top roll. The surface of the cast sheet mirrors the surface condition of the rolls and it is therefore necessary to intermittently machine the bottom roll to restore its surface and maintain sheet quality. When both rolls are driven by a single motor, this necessitates machining sound metal from the top roll to maintain uniform diameter of the two rolls. This unnecessarily shortens the life of the shell on the roll. This is avoided with a dual drive caster where the two rolls are independently driven. Light machining may be all that is required for dressing the surface of the top roll, and its shell may last much longer. This invention concerns control of casting speed in such a dual drive casting machine.
To maximize production rate, it is generally desirable to cast metal at the highest possible speed. The speed depends on many variables, including the width and thickness of the sheet being cast, the alloy being cast, roll surface condition, roll temperature, molten metal temperature, tension applied by the winder, and the like, as is well known to those operating such machines. A problem sometimes encountered when a caster is operated at too high a speed is sticking of the metal to the roll surface. Such sticking is intolerable since the sheet surface is damaged to the extent that the sheet is unusable. It is usually desirable, however, to operate the roll caster near the speed at which sticking may occur to maximize production.
A phenomenon known as microsticking has been observed. This seems to be temporary sticking in minor areas and is believed to be a precursor of more severe sticking, which is to be avoided. The usual remedy when microsticking or sticking occurs is to reduce roll speed until the problem is cured. It may thereafter be feasible to increase roll speed as operating variables change, to regain some or all of the former production rate.
Traditionally, casting machine operators have observed a variety of operating parameters for controlling casting. Such machines have been controlled manually with the operator observing motor current, roll separating force, metal temperature, roll current water temperature, sheet quality, etc. for controlling casting machine operating parameters, including speed. One important such parameter has been the casting machine motor current. An operator typically maintains a selected motor current for uniform operation. Sticking results in an increase in motor current to maintain casting speed and can be detected by observing current.
U.S. Pat. No. 4,501,315 describes a method of controlling a casting machine to avoid adhesion of the metal to the rolls. The method compares the frequency of variations of torque on one of the rolls with a reference frequency. When the variation frequency is greater than the reference frequency, operating parameters are changed to reduce the variation frequency.
It is desirable to provide a technique for controlling a roll caster based on parameters other than the frequency of variations of torque. It is particularly desirable to provide a technique appropriate for a dual drive casting machine. It is also desirable to provide a control technique that maximizes casting speed.